In order to suppress electromagnetic interference (EMI) of AC-DC switched mode power supplies (SMPS), in most cases an input filter is required on their input side. In addition to an inductance, EMI filters typically include one or more capacitors connected between the mains terminals. These capacitors are also known as X-caps. EMI filters typically also include one or more capacitances connected between one of the mains terminals and a protective Earth (which are also known as Y-caps). Typically, this protective Earth takes the form of a secondary ground, to which the Y-caps are connected, whereas typically the bridge rectifier is grounded to a separate primary ground; the primary and secondary grounds have mains separation, but may typically be connected by one or more further Y-caps.
It is desirable, and in some regulatory regimes it may be mandatory, that the voltage between the mains terminals of the switched mode power supply is reduced to a safe value within a certain time after the mains is unplugged. Otherwise, there remains a risk of electrical shock to the user, by inadvertently contacting the terminals of the plug.
As examples, according to the international Safety Requirement standard. IEC600665 for audio, video and similar electronic apparatus, it is required that within 2 seconds of a power supply being unplugged, the voltage between the terminals should be less than 60 V. Further, according to IEC60950, the decrease should occur within one second. When the mains supply to the SMPS is disconnected, for instance by the mains being unplugged, residual charge on the X-caps and Y-caps may initially result in a voltage which is above the safe level.
If the power supply is operating and fully loaded, the load will in most instances rapidly result in the discharge of the charge on the capacitors. However, under no-load conditions, the power supply may have been disabled or disconnected from any load; also, under conditions where the mains voltage has fallen below acceptable levels, (so-called “brownout” conditions), the power supply may have been switched off. Under such conditions, capacitances on the output side of the rectifier may prevent discharge of the x-cap and y-cap capacitors, on the input side of the rectifier, to an acceptable level within an acceptable period.
Thus, further measures may be required to ensure the capacitors are appropriately discharged under such conditions. It is known to provide an additional resistor discharge network in parallel with the mains input, in order to allow for such discharging. In one known configuration, the discharge resistors are adapted to also be used as sense resisters, to measure the instantaneous mains voltage, for brownout protection.